Wai-Lun Man

Chemical and Visible‐Light‐Driven Water Oxidation by Iron Complexes at pH 7–9: Evidence for Dual‐Active Intermediates in Iron‐Catalyzed Water Oxidation

In recent decades, tremendous efforts have been devoted by chemists to develop efficient, cost-effective catalytic methods for solar-driven water oxidation, which would provide an unlimited source of protons and electrons for the production of hydrogen and other renewable fuels. However, to be economically viable, the catalysts for water oxidation should be made from earth-abundant materials; so far only a few cobalt, manganese, iron, and copper water-oxidation catalysts (WOCs) have been developed. Among the first-row transition metals, iron is probably the most desirable to be used in WOCs because it is the most abundant and relatively nontoxic. Collins, Bernhard, and co-workers recently reported the use of an iron(III) complex bearing a tetraamido macrocyclic ligand (Fe-TAML) to catalyze water oxidation by Ce at approximately pH 1 with a turnover number (TON) of 18 and turnover frequency (TOF) of greater than 1.3 s . Subsequently, Fillol and Costas et al. , also reported that a number of iron complexes bearing tetradentate Ndonor ligands can catalyze water oxidation at low pH with TON> 350 and > 1000 using Ce and IO4 , respectively. Herein, we report chemical and light-driven water oxidation catalyzed by a number of iron complexes and iron salts at pH 7–9 in borate buffer. We provide evidence that at this pH range, Fe2O3 particles are produced, which are the actual catalyst for water oxidation. The catalytic activity of various iron complexes towards water oxidation at pH 7–9 was investigated by a chemical method using [Ru(bpy)3](ClO4)3 (bpy = bipyridine) as the terminal oxidant (Table 1). [Ru(bpy)3] 3+ is commonly used as an oxidant in this pH range because of its relative stability and high redox potential (E = 1.21 V), whereas Ce, the other commonly used oxidant, is stable only at low pH values. Initially we investigated the complex cis-Fe(mcp)Cl2 (mcp = N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)cyclohexane-1,2-diamine; Figure S1), because it was recently reported to be a highly active catalyst for water oxidation by Ce or IO4 at low pH. In our hands, when we used this complex as a catalyst and (NH4)2Ce(NO3)6 as the oxidant in unbuffered water, we obtained a TON of 290 15, in reasonable agreement with the value of 320 15 reported in the literature. However, when we used [Ru(bpy)3](ClO4)3 as the oxidant at pH 7–9 in borate buffer, no oxygen was produced (after subtracting the background signal) from this complex (Table 1, entry 2 and Figure S2). On the other hand, when other iron complexes such as [Fe(bpy)2Cl2]Cl, [Fe(tpy)2]Cl2, cis-[Fe(cyclen)Cl2]Cl, and trans-Fe(tmc)Br2 (where tpy = 2,2’:6’,2’’-terpyridine, cyclen = 1,4,7,10-tetraazacyclodecane, and tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) were used as catalysts, oxygen evolution readily occurred, with TON ranging from 19 to 108 (Table 1, entries 3–6). Notably, simple iron salts such as Fe(ClO4)3 is even more active than the other iron complexes (Table 1, entry 7; Table S1 and Figure S3). The maximum TOF of 9.6 s 1 is higher than those of other earth-abundant artificial catalysts such as [Co4(H2O)2(aPW9O34)2] 10 (5 s ) and Fe-TAML (1.3 s ). In the absence of an iron catalyst, 8% yield of oxygen was also detected (Table 1, entry 1), which comes from background oxidation of water by [Ru(bpy)3] . No oxygen could be detected when the reaction was carried out in phosphate buffer (Table S1, entry 8), which is attributed to the formation of the very insoluble FePO4 (Ksp = 9.92 10 ). On the other Table 1: Iron-catalyzed water oxidation by [Ru(bpy)3](ClO4)3 at pH 8.5 in borate buffer.

Efficient catalytic oxidation of alkanes by Lewis acid/[Os(VI)(N)Cl4]- using peroxides as terminal oxidants. Evidence for a metal-based active intermediate.

The oxidation of alkanes by various peroxides ((t)BuOOH, H2O2, PhCH2C(CH3)2OOH) is efficiently catalyzed by [Os(VI)(N)Cl4](-)/Lewis acid (FeCl3 or Sc(OTf)3) in CH2Cl2/CH3CO2H to give alcohols and ketones. Oxidations occur rapidly at ambient conditions, and excellent yields and turnover numbers of over 7500 and 1000 can be achieved in the oxidation of cyclohexane with (t)BuOOH and H2O2, respectively. In particular, this catalytic system can utilize PhCH2C(CH3)2OOH (MPPH) efficiently as the terminal oxidant; good yields of cyclohexanol and cyclohexanone (>70%) and MPPOH (>90%) are obtained in the oxidation of cyclohexane. This suggests that the mechanism does not involve alkoxy radicals derived from homolytic cleavage of MPPH but is consistent with heterolytic cleavage of MPPH to produce a metal-based active intermediate. The following evidence also shows that no free alkyl radicals are produced in the catalytic oxidation of alkanes: (1) The product yields and distributions are only slightly affected by the presence of O2. (2) Addition of BrCCl3 does not affect the yields of cyclohexanol and cyclohexanone in the oxidation of cyclohexane. (3) A complete retention of stereochemistry occurs in the hydroxylation of cis- and trans-1,2-dimethylcyclohexane. The proposed mechanism involves initial O-atom transfer from ROOH to [Os(VI)(N)Cl4](-)/Lewis acid to generate [Os(VIII)(N)(O)Cl4](-)/Lewis acid, which then oxidizes alkanes via H-atom abstraction.

Cerium(IV)-Driven Water Oxidation Catalyzed by a Manganese(V)–Nitrido Complex†

The study of manganese complexes as water-oxidation catalysts (WOCs) is of great interest because they can serve as models for the oxygen-evolving complex of photosystem II. In most of the reported Mn-based WOCs, manganese exists in the oxidation states III or IV, and the catalysts generally give low turnovers, especially with one-electron oxidants such as Ce(IV) . Now, a different class of Mn-based catalysts, namely manganese(V)-nitrido complexes, were explored. The complex [Mn(V) (N)(CN)4 ](2-) turned out to be an active homogeneous WOC using (NH4 )2 [Ce(NO3 )6 ] as the terminal oxidant, with a turnover number of higher than 180 and a maximum turnover frequency of 6 min(-1) . The study suggests that active WOCs may be constructed based on the Mn(V) (N) platform.

A novel tricyanoruthenium(III) building block for the construction of bimetallic coordination polymers

Reaction of excess cyanide with a ruthenium(VI) nitrido complex bearing a tridentate Schiff base ligand produces a novel tricyanoruthenium(III) complex in which nucleophilic substitution of an imine hydrogen of the Schiff base by cyanide has occurred, this complex is a useful building block for the construction of 3d-Ru(III) magnetic materials.

C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex.

We report experimental and computational studies of the facile oxidative C-N bond cleavage of anilines by a (salen)ruthenium(VI) nitrido complex. We provide evidence that the initial step involves nucleophilic attack of aniline at the nitrido ligand of the ruthenium complex, which is followed by proton and electron transfer to afford a (salen)ruthenium(II) diazonium intermediate. This intermediate then undergoes unimolecular decomposition to generate benzene and N2.

Reaction of an Osmium(VI) Nitrido Complex with Cyanide: Formation and Reactivity of an Osmium(III) Hydrogen Cyanamide Complex

Reaction of [Os(VI)(N)(L(1))(Cl)(OH(2))] (1) with CN(-) under various conditions affords (PPh(4))[Os(VI)(N)(L(1))(CN)(Cl)] (2), (PPh(4))(2)[Os(VI)(N)(L(2))(CN)(2)] (3), and a novel hydrogen cyanamido complex, (PPh(4))(2)[Os(III){N(H)CN}(L(3))(CN)(3)] (4). Compound 4 reacts readily with both electrophiles and nucleophiles. Protonation and methylation of 4 produce (PPh(4))[Os(III)(NCNH(2))(L(3))(CN)(3)] (5) and (PPh(4))[Os(III)(NCNMe(2))(L(3))(CN)(3)] (6), respectively. Nucleophilic addition of NH(3), ethylamine, and diethylamine readily occur at the C atom of the hydrogen cyanamide ligand of 4 to produce osmium guanidine complexes with the general formula [Os(III){N(H)C(NH(2))NR(1)R(2)}(L(3))(CN)(3)](-) , which have been isolated as PPh(4) salts (R(1) = R(2) = H (7); R(1) = H, R(2) = CH(2)CH(3) (8); R(1) = R(2) = CH(2)CH(3) (9)). The molecular structures of 1-5 and 7 and 8 have been determined by X-ray crystallography.

Synthesis and antitumor activity of a series of osmium(VI) nitrido complexes bearing quinolinolato ligands

A series of osmium(VI) nitrido complexes supported by quinolinolato ligands have been prepared and they exhibit promising in vitro anti-cancer activities. These results establish that Os(VI)≡N is a potentially versatile and promising platform for the design of a variety of high-valent anti-cancer drugs.

Functionalization of Alkynes by a (Salen)ruthenium(VI) Nitrido Complex

Exploring new reactivity of metal nitrides is of great interest because it can give insights to N2 fixation chemistry and provide new methods for nitrogenation of organic substrates. In this work, reaction of a (salen)ruthenium(VI) nitrido complex with various alkynes results in the formation of novel (salen)ruthenium(III) imine complexes. Kinetic and computational studies suggest that the reactions go through an initial ruthenium(IV) aziro intermediate, followed by addition of nucleophiles to give the (salen)ruthenium(III) imine complexes. These unprecedented reactions provide a new pathway for nitrogenation of alkynes based on a metal nitride.

Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex.

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.

Novel heterobimetallic ruthenium(III)–cobalt(II) compounds constructed from trans-[RuIII(Q)2(CN)2] (Q = 8-quinolinolato): synthesis, structures and magnetic properties

Reaction of [Ru(II)(PPh(3))(3)Cl(2)] with HQ and KCN produces a new dicyanoruthenium(III) building block, [Ru(III)(Q)(2)(CN)(2)](-). It reacts with hydrated CoCl(2) in MeOH or DMF to produce a trinuclear compound 2 or a 1-D zigzag chain 3.